This invention relates to additives in growth media used for cultured cell lines. In particular, the invention relates to mammalian cell growth enhancers and antibody production stimulators comprising a unique series of modified or derivatized prepolymers and polymers. The prepolymers are formed from isocyanate end-capped monomers and polyols which are substantially ethylene oxide based units. Modification is accomplished by contacting the prepolymer with a reactive compound having at least one NCO-reactive group. NCO-reactivity can be found in sulfhydryl (--SH), amine (--NH.sub.2), or carboxyl (--COOH) groups. Upon modification of the NCO groups, the derivative compounds express an additional "free" functional group and are characterized by cell growth enhancement properties. This is particularly so when the free functional group on the modified prepolymer or polymer is either a sulfhydryl, (--SH), a sulfonic acid group (SO.sub.2 OH) or a free amino (--NH.sub.2) In some instances, e.g. when a sulfhydryl or sulfonic acid is expressed, cellular antibody production is stimulated. Further, a modified prepolymer or polymer may express two or more of the above functional groups. The final form of the claimed cell growth enhancer and/or antibody production stimulator can be in the form of modified prepolymer, modified polymeric units from polymerization of the modified prepolymer or modified polymeric units comprising the dissolution product from polyurethane gels formed from the modified prepolymer.